


Mental Illness ||Reader x UF Sans (Red)||(SLOW UPDATES)

by Beautybellalove



Category: Underfell AU - Fandom, Undertale (Video Game)
Genre: Alternate Universe - Underfell (Undertale), Angst and Feels, F/M, Falling In Love, Fluff and Angst, I need a better name help, I used way more tags here, Original Character(s), Reader Is Not Frisk (Undertale), Reader and cat, Reader has a sorta evil sister, Reader seems insane, Reader w/ DID, Reader w/ depression, Reader w/ dissociative identity disorder, Reader w/ mental illness, Reader x UF Sans (Red), Reader x UF Sans - Freeform, Slow Burn, Underfell Alphys (Undertale), Underfell Asgore Dreemurr, Underfell Asriel Dreemurr, Underfell Flowey (Undertale), Underfell Grillby (Undertale), Underfell Mettaton (Undertale), Underfell Papyrus (Undertale), Underfell Sans (Undertale), Underfell Sans Needs a Hug (Undertale), Underfell Toriel (Undertale), Underfell Undyne (Undertale), Underfell x reader, You're in love, reader has a sister, red x reader - Freeform, y/n
Language: English
Status: In-Progress
Published: 2020-10-15
Updated: 2021-01-13
Packaged: 2021-03-08 18:48:28
Rating: Not Rated
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 3
Words: 2,184
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/27021529
Author URL: https://archiveofourown.org/users/Beautybellalove/pseuds/Beautybellalove
Summary: You're looking for adventure, anything to get you out of this miserable world, so when you heard the stories of Mount Ebbot, you climb it to see what all the fuss is about. You know you can't die, but you hoped maybe some magical creature would take you away.
Comments: 10
Kudos: 6





	1. Before The Fall

**Author's Note:**

> Authors note: This goes off the idea the monsters live forever (if they're not dusted), or well, a really long time. Because heads up, you can't die. ;) There's a whole story behind it because I'm basing it off of one/two of my OC's and changing them to mold you? If that makes sense? Oh, and I'm using she/her pronouns. 
> 
> Y/n stands for 'Your name.'
> 
> S/n (not officially) stands for 'Sisters name.'

~

It was that fateful night you fell to your utter doom. It was planned, of course, you  ** wanted ** this, no, you  ** deserved ** this. A while before  _ it _ happened, you wouldn't consider yourself the type to have directed hatred toward yourself, and it tugged at your soul, it hurt to hate you, but at the same time, it's all you  _ knew. _ You were a mess, and with the pressure to be something everyone else wanted you to be, over the years, it grew too much for you to handle. So you opted to leave this ungrateful world. Though that wasn't the reasoning, not this time because It  _ never _ worked, and every time you tried, there was a price, and that was your memory. Eventually, you'd get it back, but it brought a whole bunch of unnecessary problems you didn't need. So when you heard about the stories of Mount Ebbot, it piqued your interest. 

You hitchhiked a train cart, as you usually did when you needed to get somewhere. It took almost the whole day to get there, and you weren't surprised. Though every stop, you needed to find a new place to hide, and that gradually drained your energy, hiking up a mountain wouldn't be fun. But you'd persevere like you always would; you were determined to leave, and if the stories were true, that wouldn't be a problem. That in itself gave you drilling questions, what was it like? Was it some sort of underworld? Or was it a magical realm of sorts? At that last question, you snorted out a laugh that was even too much for you, though you did wonder. "Would you  _ shut _ up?!" A voice snapped in your mind, "You can be so annoying." You could feel your sister roll her eyes. "You haven't spoken in a while. Are my thoughts really that much of a bother?" You'd question, a smirk playing at your lips. You hear her sigh loudly, almost angrily, and that was never a good thing. " **_ You _ ** are a bother." She'd state with a harsh tone. You faked a hurt expression, putting your hand on your forehead, "I see how it is!" You'd exclaim, dropping to your side dramatically. While it was clearly fake, your kitten nuzzled her head against your arm in worry. She had a pure snow-white coat, and she was about the size of your palm. You sat up with a chuckle in your voice, "Ah, I'm okay, just playing." You smiled at your feline friend, petting her softly. She purred, finding a comfortable space in your lap. 

~

You jolted awake at the sound of the train wheels screeching to a stop. You looked around frantically for another hiding place, but you couldn't find one. Panic rises within you as the boxcar doors are slowly drawn open. With a quick breath, you readied yourself for the inevitable run, and when the opportunity came, you bolted out as if your life depended on it. The men who were to transport the cargo to their next destination yelled at you to stop, but that only encouraged you to run faster, a giddy smile glued to your face. 

~ 

Halfway up the mountain, you crouched down to take a couple of greedy breaths. You had tired yourself out to the point of exhaustion (though this was nothing new) you presumed you could walk the rest of the way. Come to think of it, where was the place those who climbed disappeared to? You'd only heard, if you climbed the mountain, you'd never return, were aliens supposed to capture you in their floaty thing and make you milk their cows? How did it work? You'd look around as if that would solve your question, though you didn't see anything unusual. "Sigh, I really believed something would happen." You'd lay your cheek on your fist, a bit frustrated you'd come all this way, and there was  _ nothing _ . Dusting yourself off, you stand, deciding to take one more look around before giving up. 

Sure enough, you stumbled by a hole. From what you, saw it seemed pretty deep with some yellow fuzz at the bottom; you wondered what that was about. Before you could think about a safe way to get down, you felt a push against your own will. The next thing you knew, you were falling, you tried to summon your sister's soul magic, but it wouldn't work. You could hear her cackle from above you, but that's not what worried you. What caught your attention was your furry companion that was falling beside you; cushion or not, her tiny body wouldn't be able to withstand the fall, and it frightened you. In a frenzy, you swooped her in your arms and hugged her tightly. You prepared yourself for the impact, but it never came. You only then realized you were squeezing your eyes shut, so you blinked, now glancing around to see what stopped your fall. You could see your sister's ghostly form, smirking at you from the corner of your eye. You could tell your struggle  _ amused _ her, but putting your  _ only  _ friend's life at risk was enough to fuel you with fury. "What are  **_ you  _ ** going to do about it, Y/n?" She'd tease, sticking her tongue out at you. S/n knew you were too weak to stand up for yourself, at least, against her. You felt too much regret and guilt about the past to even dare lay a finger on her, which kept her in control for so long. If you wouldn't fight, she'd continue to pull the strings until it became boring, but that was to be determined. "Just put me down, S/n." You demanded quietly, but harsh all the same. She shrugged as if used to this by now. You examined your kitten to make sure she wasn't harmed in any way. She seemed frazzled by the whole experience, but nothing serious that could put her health at risk. You breathed a sigh of relief. 

~

Shortly after composing yourself, you stood up and decided to check this place out. You were always too curious for your own good. When you headed into the next room, you never would've thought, in a million years, you'd see what you saw that moment. 


	2. Meet Flowey

**Summary for the Chapter:**

> Y/n meets Flowey! She's not too sure about him, or this world for that matter, but she decides to go along with it, after all, it was her choice.

~

Shortly after composing yourself, you stood up and decided to check this place out. Instantly the scarlet red walls and floors caught your eye. It looked like stone, though red was an odd color for that. It was too bright, you mused. It seemed a bit _too_ natural; like you'd be seeing a lot less color and more of an edgy reddish-black tone, it screamed the 80s to you, but who were you to complain? As you continued further, you were led to a large opening. When you headed in, you never would've thought, in a million years, you'd see what you saw that moment. Well, maybe that was a bit extreme. You most likely could have imagined it, but the fact it was talking to you- and oh God, what's happening to you now?!

~

Only moments before your little freakout, a golden flower spoke to you; from what limited information you gathered by their weak and trembling form, was that they were young, at least appeared to be, vocal and physical wise. Wait, why was that a concern of yours? You weren't sure, but you felt a tinge of guilt for them; they must've suffered a lot. They begged for your mercy, and you almost wanted to give it to them, but when the very culmination of your being was ripped out from the comforts of your vessel, all traces of guilt vanished. How **_dare_** they touch you? Though you knew this wasn't you, rather your twin's thoughts, and you struggled to regain back control. From the flower's point of view, you must've looked ridiculous, as you were going back in forth with yourself, scolding the air, running back and forth, even resorting to slapping yourself in an attempt to knock yourself out to give the little bud time to escape from your sister's wrath. The flower looked at you with uncertainty but mostly fear of what would happen next. You, with a sharp slap to the head for good measure, felt out to the mercy option, still highly confused about why you needed to press something and the very fact there was that kind of option in the first place. You tried not to question it; this is what you wanted, right? The flower gazed at you, seeming slightly relieved but also confused about your actions, "D-did you just spare me?" You hear them stutter, striking your own confusion. He must've _really_ been through a lot; you could relate. "I've never seen anyone spare anything before..." They seemed to trail, though quickly adding a: "T-Thank you!" Their smile met yours, you didn't realize when you started smiling, but you couldn't help it; their relief brightened your day. "You're new in the underground, aren't cha?" He questioned, though it seemed rhetorical, considering he continued to speak, but it didn't bother you in the least. "You must be very confused!" No shit, Sherlock, you'd silently retort, a bit surprised at your rude behavior, though you could see it pleased S/n. "Don't worry. I can help you through the ruins!" You weren't sure you had a choice at this point, though it might be good to have a guide through whatever the 'ruins' were. "I know this place off the top of my head." Their smile seemed to waver at their next response. "However, you must be careful with _her_." They glanced to the side, worry evident in their features. "Unfortunately, I can't help you with that part." They looked back at you, adding a weak: "Sorry." As quickly as their mood went down, it went back up. "Well, we won't progress without moving! Let's go!" They seemed hopeful, but you could tell it wouldn't be long-lived. Maybe you shouldn't have complained so much about your life, maybe you should've made do, but could you have? Could you have escaped everything and lived a peaceful life? No, something would figure out a way to pull you down. They always did. At least here it was a fresh start; even if it proved troubling, you always managed.

**Notes for the Chapter:**

> Hello! I just wanted to say that both chapters 3 and 4 are in production! I'm trying to figure out a title, and I was thinking: "Don't give up!" but that sounds sorta cliche. I was thinking back to when the player dies; it always has some inspirational message to keep you going. One of which is 'don't give up'. While the reader technically can't die, either way, she can give in. Give in to what you may ask- well, you'll find out soon enough. =) Oh! And also, there will be some chapters I can make specifically for you all! What I mean is, if you give me a topic or a basic idea, I can try to use that to build a chapter off of it since this is still in production. Depending on how many people request certain things, I'll do a vote, and you can pick based on the top three ideas- (if I even get that many). I may pick and choose or change your idea so it can fit the story better. And I can give cameos to monster OC's you may have if you want, though I need a description of them and their personalities (I may get it wrong, though). You might even be a recurring character, sound fun?


	4. Are You Ready?

**Summary for the Chapter:**

> Are you ready for the storm that's about to come? Are you ready for the pain and suffering of those around you, and yourself? Will you be able to save these creatures? Or will you kill them all? Are you ready for the choice? Are you ready for your life to change? 
> 
> A R E 
> 
> Y O U 
> 
> R E A D Y ?
> 
> ...
> 
> Because I'm sure not.

**Notes for the Chapter:**

> Hello! I'm not dead! This chapter has been floating around in my notes for so long I thought I'd just post it. I don't have the motivation to write about Toriel. I have a bunch of ideas that I'm not sure how to put into words. But if you happen to see chapter three one of these days, go ahead and check it out. I will have little memories and or flashbacks of it throughout the story. Angy floof mom hits hard, especially for a lonely reader such as yourself. =) 

~

As you trudged out in the cold snow, you heard your sister scoff. "That was pathetic, Y/n." You rolled your eyes, focusing on getting past this area. It was dangerous, as Toriel had said. You needed to see the King as quickly as possible. Though at the same time, if these 'monsters' were as bad as the overgrown goat had described, shouldn't it be better they stayed here? You couldn't contemplate it further as you heard a twig break behind you, "Who's there?! What was that?!" Your flower friend shouted, visibly terrified. You felt fear sink onto you, but you shrugged as if there was no problem. Though when a skeleton appeared in front of you, you nearly had a heart attack. You jerked back, a bit taken off-guard, though as quickly as you had, you regained your confidence and stature. The skeleton man then introduced himself with a grin that didn't quite fit. It seemed fake, though it was hard to tell. You wanted to introduce yourself, but something stopped you. You could feel your sister's impatience growing, so when she leaped out of your body, you weren't surprised, though you did let out a startled yelp now plummeting into the snow. This confused the skeleton in front of you, "are you-" he was cut off as he felt a breeze rush past him. He strained his eyes to see a transparent outline of what looked to be a human form. Though when he blinked, it was gone. Sans then looked at the girl in the snow, creepy vibes crawling down his metaphorical back. "you need some help there, bucko?" He offered, lending a hand. Though when you took the boney hand, you felt a shock. You blinked, bemused. This earned a chuckle from Sans. "the old buzzer in the hand trick! it's always a _shocker_." You find yourself snickering at the pun, "T-That was terrible." You add, still a bit light-headed from the prior experience. He seemed unphased; this only inflating his ego. There was a moment of awkward silence. You notice small trickles of red sweat bead at his forehead. You then see him scratch his head, glancing away from you. "y'know, i'm supposta' be on the watch for humans, right now... you see my brothers a human hunting _fanatic._ you'd be killed if he were to see you." _This new information was heavy, holy crap. Weren't you two laughing a minute ago? What happened?_ "and then your corpse would be dragged to the king." He shutters at the mental image he mistakenly gave himself, though if you weren't paying close attention, you might've missed it. You decided not to retort _outwardly_ and silently listened to the skeleton, no matter how much snow you got in your pants. 

~

**Notes for the Chapter:**

> Hopefully, with this published, I'll be able to relax. I couldn't continue forward because of the stress of getting the Toriel interaction done. But now that this is out, I can continue forward. u>u Thank you to those who stuck around! It means a lot. QwQ 

**Author's Note:**

> This is also on:
> 
> Quotev: Beautybellalove (Izzy23767)
> 
> Wattpad: Beautybellalove
> 
> Undertale Amino
> 
> Will be on:
> 
> YouTube: Beautybellalove101 (sneak peeks and updates)
> 
> I also have other stories besides Undertale in the making if you wanna check those out. ^^ (When I put them on there lol).


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